1. Field of the Invention
The present invention relates to a method and a circuit for detecting a carrier-to-noise ratio (CN ratio), and more particularly to a CN ratio detecting circuit used for adjusting a satellite broadcast receiver at an installation of an antenna and also for checking receiving conditions of satellite broadcast signals.
2. Description of the Related Art
FIG. 1 shows a block diagram of a prior art CN detecting circuit for a satellite broadcast signal. In the figure, variable resistor 1 is directed to adjusting the input level of a frequency-demodulated signal FD of a received satellite broadcast signal. Wide-band amplifier 2 has a pass-band width wider than 10 MHz. The noise component included in the output of wide-band amplifier 2 passes through band-pass filter (BPF) 15 with a center frequency of approximately 9 MHz, and is amplified by noise amplifier 16. The gain of noise amplifier 16 is adjusted in advance with variable resistor 17 so that the value of the CN ratio to be determined has a predetermined relation to a detected noise level. The output level of noise amplifier 16 is next detected through smoothing or averaging by noise detector 18, and then converted to a digital noise signal by analog-to-digital (A-D) converter 19. A-D converter 19 is typically a 4 bit converter capable of representing the converted digital signal in 16 steps. Microcomputer 20 calculates the CN ratio in accordance with a predetermined program from the digital noise signal and delivers the calculated result to a given display device.
Wide-band amplifier 2 also provides an output OF, to be delivered to a transmission line. The output of wide-band amplifier 2 is converted so that the output impedance matches with the 75 .OMEGA. characteristic impedance of the transmission line by means of impedance converting circuit 21 which transmits the impedance-matched output to the transmission line as the frequency-demodulated output OF.
The problem encountered in the CN ratio detecting circuit described above is that, because of dispersions in the gain and of the signal-to-noise ratio of both the noise amplifier and the noise detector, and also because of errors caused by temperature variations and non-linear distortions in the circuit characteristics, it is difficult to adjust the gain of the noise amplifier so as to cause the detected noise level to have a predetermined correlation with the CN ratio of the frequency-demodulated signal. This involves a difficulty in obtaining a precise CN ratio through analog processing of the received signal, which results in a difficulty in adjusting and checking the satellite broadcast receiving system.
Another problem encountered is that using expensive parts such as an A-D converter, in the analog CN-ratio detecting circuit brings about an increase in the cost of the circuit.